Superplastic forming of aluminum, titanium and other metal parts is widely practiced especially in the aerospace industry. The process includes placing a sheet of metal having superplastic characteristics between a die lid and a die base, heating the die and the captured sheet of metal to a temperature at which the metal exhibits superplastic characteristics, applying force to the die lid to hold it closed on the die base against the gas pressure which will be applied against the metal inside the die, and applying the gas pressure to cause the metal to stretch into the die cavity in the base and conform to the surface of the die cavity which is the shape of the final part. After forming, the die lid is removed and a part is cooled and removed from the die base cavity.
A long term problem in the use of the superplastic forming process which has received many attempts over the years at a solution is the excessive thinning of the part in certain areas such as the lower inside corners of concaved parts. Excessive thinning of the part in localized areas such as this can make the part unacceptable and require expensive solutions such as making the part in two pieces and welding the pieces together, or making the part with material that is thicker than necessary just to attain the required thickness at the corners or other areas that experience excessive thinning.
One known technique for minimizing thickness when forming superplastic material onto a convex die is to first expand the metal blank into a cavity in the lid to preform the blank so that when the pressure is reversed, the blank is formed downwardly over the convex mold in the die base. This technique improves the thickness uniformity but does not solve the problem of localized thinning in corners of deep concave dies or thinning around tall thin convex forms. Other processes are available which require multiple processing of the blank which increases the handling cost and can result in undesirable metallurgical characteristics because of the multiple heating cycles. Thus, the art has long sought a process by which the thickness of the part in particular areas of concern can be tailored to provide either uniform thickness throughout the entire part, even in areas where uniform thickness has not been possible in the past, or localized area of thickness on parts which needs strengthening in particular areas of the part.